1. Field of the Invention
The present invention relates to a power transmission arrangement for a vehicle.
2. Description of the Related Art
An automatically and hydraulically controlled wet multiplate clutch is well known in the art for transmitting a drive force or power of an engine to a transmission. As illustrated in FIG. 6 of the accompanying drawings, this type of clutch generally includes a plurality of clutch plates (i.e., drive plates 114 and driven plates 111) housed in a clutch housing 116, and a clutch piston 120 for pushing the drive and driven plates against each other to engage the clutch. An oil pressure controlled to an appropriate value is fed to the clutch piston 120 through an oil passage 119 as indicated by the arrow. When the oil pressure is released, the clutch piston 120 is returned back by a return spring 121 such that the engaging force no longer acts on the clutch plates to disengage the clutch.
The clutch housing 116 is an input side of the clutch, and an input shaft 118 of the clutch is operatively connected to a crankshaft (not shown) of an engine to obtain a power from the engine. An output shaft 113 rotatably extends in the clutch housing 116. The output shaft 113 is an output side of the clutch and operatively coupled to an input shaft (not shown) of a transmission. The drive plates 114 are mounted on the clutch housing 116, and the driven plates 111 are attached to the output shaft 113 via a clutch holder 112. The clutch plates are mounted or attached to the associated parts by way of splines.
Referring to FIG. 7 of the accompanying drawings, illustrated is an oil pressure feed system. An oil pump 125 is driven by the engine to draw an oil from an oil tank 126 and feed the oil toward the clutch piston 120 and other components to be lubricated. On its way, the oil is controlled in flow rate and pressure by adjustable (or variable) metering valves 127 and 128. Feeding and releasing of the oil to and from the clutch piston 120 are determined by an electromagnetic switching valve 129. A reference numeral 130 designates a relief valve.
In the wet multi-plate clutch, the clutch plates are included in the above-mentioned "other components to be lubricated", so that the lubrication oil is fed to the clutch plates. This lubrication oil feeding is necessary to allow friction heat generated upon clutch engagement to escape via the oil. In this manner, the clutch plates are oil cooled. Therefore, it is possible for the clutch to slip in a longer period in the wet multi-plate clutch than a dry friction clutch.
If the wet multi-plate clutch is employed and an automatic clutch system for automatic clutch engaging and disengaging is incorporated in a vehicle, it is possible to cause the clutch to slip for a relatively long period during vehicle starting so that it is in turn possible to select a relatively high gear position from the beginning.
However, if the gear position is too high at the starting or the starting with a high gear position is repeatedly experienced, the clutch will soon be deteriorated.
The present assignee proposed an arrangement which combines a torque converter with a wet multi-plate clutch (Japanese Patent Application, Laid-Open Publication No. 10-184726). A shock generated upon automatic clutch engagement is absorbed in this arrangement, so that complicated clutch engagement control is not needed during the starting (particularly sudden starting or rapid acceleration). However, even if this arrangement is employed, an excessively large load acts on the torque converter if the starting with a high gear position is repeated. As a result, the torque converter oil and/or torque converter itself will be damaged in a reduced period. In addition, if the same oil is commonly used as the clutch lubrication oil and the torque converter oil, deterioration of the torque converter oil promotes deterioration of the clutch.